1. Field of the Invention
This invention relates to an optical signal receiver for detecting optical signals transmitted from a remotely opposed transmitter and also to an optical space transmission system comprising such a receiver and adapted to convey information to it.
2. Related Background Art
With a conventional optical space transmission system, a signal to be transmitted from a transmitter is modulated into an optical signal, which is then transmitted from the transmitter to a receiver by way of the atmosphere. Upon receiving the signal, the receiver demodulates the optical signal transmitted from the transmitter so that the information that the signal represents can be conveyed from the transmitter to the receiver by way of the atmosphere.
However, an optical space transmission system using a light beam as an optical signal has a disadvantage that the optical signal can be affected and attenuated by natural phenomena such as rain falls and fogs in the atmosphere. As means for coping with this problem, it is a common practice to detect the DC component level of the light beam received by the receiver in order to judge if the optical signal receiver is operating properly to receive the optical signal transmitted from the remote transmitter or not.
When abnormal communications arise in such a conventional space transmission system due to a natural phenomenon such as a rain fall or a dense fog in the atmosphere operating as transmission paths of light beams, some of the optical receivers of the system may be forced to keep on outputting incorrect signals as long as the natural phenomenon continues. Then, as a result, the computer network devices receiving the output signals from such optical receivers can go down such that the devices may have to be initialized in order to restart their operations. Then, even if the optical space communication is returned to normal during the initialization, the device cannot resume its operation at least during the initialization period.
Additionally, when an optical transmission system has backup routes so that some or all of the transmission paths for optical signals in an optical transmission system may be switched to respective wired transmission paths when abnormal communications arise on those transmission paths, the computer network devices connected to the switched transmission paths cannot decide to switch the transmission path on the basis of the presence or absence of a bit stream received from the optical receiver so that they are often accompanied by a problem of difficulty of switching the transmission path and an interrupted telecommunication.